Site Architecture

I have a question about rules operating across entirely separate Drupal 7 sites.

I am designing a client's application as N (N < 6) separate Drupal 7 sites for perforamnce, scalability, and security reasons. Traditional Muliti-site is not an option as they need the option to place different sites on entirely different servers. (see footnote) The sites are linked by 1) pushing content via the Deploy module (i.e. a backend 'publishing' workflow), 2) Single Sign On (with e.g. the Bakery module), and 3) shared business logic.